Out of the comfort and into the fire went the central United States this week as the latest NotReallyAPolarVortex left in a hurry. Temperatures themselves are hot enough — topping 105°F in places — but as the old phrase goes, it's not the heat that gets you, it's the humidity. On days like today, the heat index is crucial.

The heat index is also called the "apparent temperature," or how hot the air actually feels to your body. Our sweat is able to cool us off through evaporative cooling. When there's a high level of moisture in the air, it's harder for the sweat to evaporate, making it harder for our bodies to cool off. Since the high humidity won't let us cool off effectively, a relatively cooler air temperature has the same effect as a much, much hotter temperature.

A few months back I argued that the dew point is a much better measure of moisture in the air than relative humidity. The dew point is the temperature at which the water vapor in the air will condense and the humidity will reach 100%. Dew points below 60°F are comfortable, while those above 65°F are uncomfortable; readings above 70°F are oppressive.


Here's a great example of humidity versus dew point: at 4:00 yesterday afternoon, it was 102°F in Topeka, Kansas with a relative humidity of just 35%. That seems really low, but it's deceiving. The dew point at the same time was sitting at 69°F, which is approaching the point at which it's unbearably uncomfortable outside.

An air temperature of 102°F is dangerous enough for people to stay outside too long, but when you factor in the amount of moisture in the air, it's too dangerous for anyone to stay outside for more than few minutes at a time. Combined with the actual temperature, that 69°F dew point results in a heat index of 111°F.


A heat index of 111°F means that the heat and moisture in the air are keeping you from cooling off to the point that the mere 102°F reading has the same effect on your body as standing outside when it's 111°F.

The National Weather Service stresses that heat indices are only valid for shady areas, and that exposure to full, intense sunshine bumps up the apparent temperature by 15°F, putting people outside at even greater risk for medical emergencies.

The heat index explains why summer in the southwestern United States is so much more tolerable than summer in the corn fields of Iowa or in the middle of Alabama. A 100°F day in Phoenix is warm but it's not oppressive because there's usually very little moisture in the air, allowing you to sweat and cool off relatively easily.

National Weather Service offices across the country issue heat advisories and excessive heat warnings when heat indices get to the point where it's dangerous for even healthy individuals to do much outside during peak heating. The heat indices required for the issuance of these products vary from office to office based on how often that particular region experiences extreme heat; ignoring the population differences, a 105°F heat index in New York City has the potential to adversely affect many more people than a 105°F heat index in Mobile, Alabama.

I took a look at some of the nastiest observations I could find in yesterday's bullseye for the heat. Across portions of Iowa and Missouri, there was a pretty large patch of dew point readings sitting around or above 80°F. That's about as high as the dew point can get in a natural setting, and it's usually due to a mouthful called "evapotranspiration," or the water evaporating out of plants; it's corn sweat.

I looked around at airport observations for the areas in the dew point maximum, and found the following grossness.

Lamoni, Iowa recorded an air temperature of 89°F with a dew point of 79°F. Now, 89°F usually isn't too bad, but the incredible amount of moisture in the air made it unbearable. The moisture caused that mere 89°F to have the same effect on your body as an air temperature of 109°F.

Likewise, look at Chillicothe, Missouri. An air temperature of 94°F with a dew point of 80°F had the same effect on the human body as an actual air temperature of 114°F. 94°F on a moderately humid day can cause serious illness or death in demographics such as the elderly or construction workers if they're not in a cool building and/or properly hydrated, but 94°F with a dew point of 80°F is a completely different ball game. Dry 94°F is much, much different from "wet" 94°F.

For the heck of it, I threw in Phoenix, Arizona as the last row on that chart. Yesterday evening the city reported an air temperature of 112°F and a pretty comfortable dew point of 44°F. The air is so dry — a 112/44 combo produces a relative humidity of about 10% — that the heat index was five degrees lower than the actual air temperature. Of course, 107°F isn't "cool," but it's illustrative of the larger point.

According to Wunderground, on July 13, 1995 Appleton, Wisconsin saw what is probably the highest heat index ever recorded in the United States. The air temperature reached 101°F and the dew point hit an astounding 90°F, leading to a heat index of 148°F. The article also notes that the "absolute highest dew point" ever recorded in the world was 95°F in Dhahran, Saudi Arabia, which, with an air temperature of 108°F, produced a "theoretical" heat index of 176°F.

Could meteorologists find a better way to convey the amount of moisture in the air relative to the actual air temperature? It probably could be done. We have simplified scales for everything else. The Enhanced Fujita Scale estimates the strength of tornadoes based on the damage they produce. We have the flawed Saffir-Simpson Scale to convey the wind speeds in a hurricane. We even have a 0-5 scale to communicate the risk for damaging severe thunderstorms on any given day.

But in the meantime, we've got the heat index, and it does a fairly good job conveying how dangerous the current temperature is when you factor in the amount of moisture in the air. Take heat advisories and warnings seriously and, for the love of common sense, don't leave your children or pets in the car.

[Images via AP, University at Albany, SimuAWIPS with annotations by the author, chart of temps by the author]

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